My son has an iPhone 3GS and he told me tonight that for a while it has been randomly vibrating. When it happens there is an icon on the screen like you are adjusting the volume up or down. Nothing seems to make it happen it just does it randomly. It will be short vibrations lasting less that a second but there are several of them and it just comes and goes. I just did a restore and it did not seem to help. Is it safe to assume this is a hardware issue?